1. Field of the Invention
The present invention relates to a control circuit and a control method of current mode control type DC-DC converter, and more particularly to prevention of subharmonic oscillation in the event of on-duty exceeding 50%.
2. Description of the Related Art
A current mode control type DC-DC converter for duty control by detecting the current flowing in a choke coil and turning off and controlling the main switching transistor is likely to induce subharmonic oscillation when the on-duty exceeds 50%. FIG. 11 is a diagram of a conventional current mode control type DC-DC converter 100. FIG. 11 is a circuit diagram of a general step-down DC-DC converter of current mode control type. A DC-DC converter control circuit 101 is a control circuit for controlling the entire DC-DC converter 100. The DC-DC converter 100 comprises a main transistor FET1, a choke coil L1, a synchronous rectifying transistor FET2, a flywheel diode D1, a capacitor C1, and a current sense resistor Rs1.
The operation is explained. The main transistor FET1 conducts when the peak value of coil current IL flowing in the choke coil L1 reaches a specified value. This operation is executed by comparing the output voltage of error amplifier ERA1 and output voltage of voltage amplifier AMP1 by voltage comparator COMP1. The main transistor FET1 does not conduct in a specified period depending on the output signal from oscillator OSC. That is, the main transistor FET1 is turned off regardless of the coil current IL.
Related technologies are disclosed in Japanese unexamined patent publication No. 2004-248374, Japanese unexamined patent publication No. 2004-40856, Japanese unexamined patent publication No. H11 (1999)-41924, and Japanese unexamined patent publication No. 2003-244953.